2. Recent HRIBF Research -
Measurement of
the 26Al(d,p)27Al
Reaction for the Study of the Astrophysical 26
Al(p,
γ)27Si Rate
[S. D. Pain (Univ. of the West of Scotland),
spokesperson]

Astronomical gamma-ray mapping, by charting the distribution of
specific isotopes, yields information which can constrain the rate of
explosive nucleosynthesis events (such as novae and supernovae) within
the galaxy. One of the landmarks in observational astronomy has been
the detailed galactic mapping of the decay of 26Al, achieved by the
observation of the 1809-keV gamma rays emitted following its beta
decay using the COMPTEL instrument aboard the Compton Gamma Ray
Observatory (see Fig.2-1), and more recently the European Space
Agency's INTEGRAL satellite. With a half-life of ~105 years, the
distribution of 26Al provides an insight into the galactic
nucleosynthesis over a timescale of about the last million years. The
mechanisms contributing to the formation and destruction of 26Al are
consequently of direct interest to the interpretation of these
gamma-ray maps [2].

Figure 2-1: COMPTEL galactic map of the 1809-keV gamma-ray line from the
decay of 26Al [1].

In most astrophysical environments, 26Al is destroyed via the
26Al(p,γ)27Si reaction. Understanding states near the proton threshold
in 27Si is crucial for constraining this reaction rate, and thus for
elucidating the amount of 26Al which survives to enrich the
interstellar medium. The difficulty in measuring directly the
strengths of these resonances, due to their small cross sections,
means that indirect approaches are required. Furthermore, due to the
difficulties inherent in measuring proton transfer reactions, an
alternative is to measure mirror states in 27Al to
obtain information about the 27Si
structure.

The 26Al(d,p)27Al
reaction has been measured in inverse kinematics at the HRIBF, in
order to study states in 27Al which are mirror
to those in 27Si. A batch-mode beam
of 26Al, of typically 5 million particles per
second, impinged on a ~150 mg/cm2
CD2 target for ~5 days. Proton ejectiles were
detected in the SIDAR and ORRUBA silicon detector arrays, subtending
angles from ~90-165 degrees in the laboratory frame. These data
represent the first measurement performed with a complete barrel of
ORRUBA. Elastic scattering was monitored in ORRUBA detectors mounted
at angles just forward of θlab = 90°. A forward array for recoil
tagging, comprised of annular segmented silicon detectors of Micron
QQQ2 design, covering angles from ~1.5 to 10 degrees, was used to
detect 27Al ions coincident with (d,p) protons.

Figure 2-2 shows an online spectrum from a single strip of SIDAR, for a
subset of the data taken during the experiment. Transitions at these
backward angles are strong candidates for ℓ = 0 transfer, pending a
full analysis of angular distributions. These data are under analysis
by, and will contribute to the PhD thesis of Stephanie Brien from the
University of the West of Scotland.

Figure 2-2: Proton energy spectrum from the 26Al(d,p)27Al reaction, for a single SIDAR strip, for a subset of the data.